Bike not running properly when it rains heavy !!!!!!

[Iowcg]

It's happened to me twice. It's raining heavy and it after 3 miles it starts to lack in power then if I rev up it sounds like its coughing and back fires loudly I assume water is getting in somewhere but don't no where, or it's something todo with the wiring maybe, the first time this happened it was hammering it down and I went through a puddle than ended up the size of a small pond :-/. It ran ok but it's like it didn't have enough power to move only to run. I left it for a day tried to kick it but nothing so bumped it and started first time and ran fine!??!. Second time was tonight in the pointing rain it started to cough as I call it then started to loose power so pulled over reved it abit and every rev it back fired so turned it off and started pushing home :-/ 2 miles down the road dripping wet I kicked started it and it started and I reved it rite up and a huge bang with 2foot flame out the exhaust and rode home was abit juddery, can anyone help me? Oh and when it's not raining it runs fine. Also water might be getting in through my petrol cap not sure tho but if anyone can help I would be most thankfull
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M l poskitt